This Program Project for the Investigation of Childhood Tumors is directed toward a better understanding of cellular regulation. Emphasis is placed on model systems which examine growth and differentiation. A particular strength of the program is the collaboration among the various investigators. Individual sections are separately abstracted and involve: (1) bone marrow transplantation with reference to histocompatibility antigens not linked to HLA, autocytotoxic cells, and immunocompetence of transplant recipients; (2) suppressor cells; (3) hybrid tumor cells as a possible model immunotherapeutic agent; (4) human serum growth factor isolated from platelets; (5) murine virus transformation in vitro and in vivo; (6) regulation of RNA metabolism by growth factors and drugs; (7) DNA polymerases in differentiating cells; (8) glucocorticoid receptors in leukemia cells; (9) differentiation in erythroid culture systems.